Aliphatic polyamides represented by nylon 6 and nylon 66 have been used as an engineering plastic for many applications because they have excellent properties such as heat resistance, chemical resistance, rigidity, abrasion resistance, moldability, and the like. The majority of them are used in the field where heat resistance is required. In particular, in the case of being exposed to a high temperature over a long period of time as in automobile parts or the like, heat deterioration or oxidative deterioration of resin is of a serious problem. Though the foregoing aliphatic polyamides have a high melting point, when exposed to a high temperature of 100° C. or higher in air, they become conspicuously brittle within a short period of time. Then, in the case of being used in an application field where such long-term heat stability (hereinafter sometimes referred to as “heat aging resistance”) is required, a formulation for preventing the oxidative deterioration at a high temperature from occurring is made by various methods.
Recently, a semi-aromatic polyamide composed mainly of a polyamide obtained by polycondensation of 1,6-hexanediamine and terephthalic acid, which has a higher melting point than the conventional polyamides and which is called “6T based polyamide”, has also been started to be used as a new engineering plastic.
For example, Patent Document 1 proposes a method of ensuring the heat aging resistance regarding the 6T based polyamide. However, since the polyamide obtained by polycondensation of terephthalic acid and 1,6-hexanediamine has a melting point in the neighborhood of 370° C. exceeding decomposition temperatures of polymers, it is difficult to achieve melt polymerization or melt molding so that it cannot endure practical use. For that reason, in fact, it is the present situation that such a polyamide is used as a composition whose melting point has been decreased to a practically useful temperature range, namely to about 280 to 320° C., by copolymerizing with from 30 to 40% by mole of a dicarboxylic acid component such as adipic acid, isophthalic acid, and the like, or an aliphatic polyamide such as nylon 6 and the like. In this way, the copolymerization of a large amount of the third component (also, a fourth component according to circumstances) is certainly effective for decreasing the melting point of polymer. Meanwhile, however, lowering of crystallization rate or ultimate degree of crystallization is brought, and as a result, not only various physical properties such as rigidity at a high temperature, chemical resistance, dimensional stability, and the like are deteriorated, but lowering of productivity following prolongation of molding cycling is brought, too. Also, as compared with the conventional aliphatic polyamides, the dimensional stability by water absorption is improved slightly, but it does not reach a level on which the problem is substantially solved. Also, lowering of toughness and an increase of coefficient of water absorption of polyamide resin compositions to be caused due to blending of a copper compound for the purpose of ensuring the heat aging resistance were problematic.
As other high-melting polyamides than the 6T based polyamide, there is a semi-aromatic polyamide obtained by polycondensation of a mixture of 1,9-nonanediamine and 2-methyl-1,8-octanediamine and terephthalic acid, which is called “9T based polyamide”. With respect to the 9T based polyamide, for example, Patent Document 2 proposes a method of ensuring long-term heat stability by the addition of a copper compound and a crystal nucleating agent. However, in these compositions, though crystallinity, heat resistance, dimensional stability following water absorption, and the like are improved, there was involved such a problem that an application range is limited because of low rigidity. Also, with respect to the 9T based polyamide, the problems of lowering of toughness and an increase of coefficient of water absorption of polyamide resin compositions to be caused due to blending of the copper compound were not solved yet.
Also, as a method of improving the heat aging resistance by blending a stabilizer other than the copper compound, for example, Patent Document 3 proposes a polyamide resin composition obtained by blending a hindered phenol compound, a phosphorus based stabilizer, and a sulfur based stabilizer; and for example, Patent Documents 4 and 5 propose a polyamide resin composition obtained by blending an amine based antioxidant and a phenol based antioxidant. However, there are not described especially preferred combinations of compounds.    Patent Document 1: JP-A-63-105057    Patent Document 2: JP-A-7-228768    Patent Document 3: JP-A-6-136263    Patent Document 4: JP-A-5-5060    Patent Document 5: JP-A-2006-28327